The role of drugs (other than those used for chemotherapy and immunotherapy) in the etiology of aplastic anemia, agranulocytosis, and thrombocytopenic purpura will be investigated in a case-control study. The methods will be similar to those used in a study of agranulocytosis and aplastic anemia now in progress in Israel and Europe. All hospitals with at least 100 beds in geographical defined areas of eastern Massachusetts, Rhode Island and metropolitan Philadelphia will be enrolled. The institutions admit virtually all cases of the three dyscrasias occurring in a base population of about 10 million people; all cases of aplastic anemia and agranulocytosis will be identified prospectively for the purpose of estimating incidence rates. A proportion of cases of thrombocytopenic purpura will be identified. Incidence rates will not be estimated for this condition. The eligibility of the cases will be determined by a committee of hematologists, according to strict diagnostic criteria. Decisions will be based on relevant clinical and hematological information. All identified cases of the dyscrasias, suitable hospital controls, and, in Massachusetts, neighbor controls will be interviewed. Information will be obtained on drug use, occupational and chemical exposures, personal data, and relevant medical history. It is expected that about 70 cases of aplastic anemia, 120 of agranulocytosis, 250 of thrombocytopenia, 1760 controls, and 300 neighbor controls will be studied. The data will be used to quantify known associations between the three dyscrasias and drug use, to identify and quantify previously unsuspected associations, and to document absence of associations for commonly used drugs. Incidence rates of aplastic anemia and agranulocytosis attributable to specific drugs, and classes of drugs, will be estimated.